In many applications it is required that a guided optical wave be efficiently radiated out of an optical waveguide, and be split into a number of optical beams and simultaneously be focused at finite distances away from the optical waveguide. Here the guided optical wave can be produced by a semiconductor diode laser. In an optical interconnection system, for example, it is not only essential to produce an array of spots with uniform intensity, but also necessary to focus most optical power into the desired spots in order to reduce power loss and to suppress spurious light. In short, such optical devices must simultaneously provide the following functions: (1) radiating the guided optical wave out of the optical waveguide; (2) splitting the radiated optical wave into a number of optical waves; and (3) focusing the radiated optical wave(s) at a finite distance away from the optical waveguide.
It is well-known that a uniform grating coupler, which is fabricated on the surface of or into an optical waveguide, can be used to achieve function (1) stated in the previous paragraph, viz., it has the ability to radiate a guided optical wave out of an optical waveguide. However, such a simple device lacks the beam-splitting and beam focusing functions. For example, refer to Tamir and Peng, Appl. Phys. 14, p. 235, 1977.
A focusing grating coupler has been proposed in, for example, Ura et al, "Focusing grating for integrated optical-disk pickup device". The Transactions of IECE of Japan, Part C, vol. J68-C, No. 10, October 1985, pp. 803-810. Instead of having periodically positioned, rectilinear grating grooves as in a uniform grating coupler, a focusing grating coupler comprises a group of curved grating grooves, each of which being uniquely defined by the interference fringes between a guided optical wave and a spherical free-space optical wave. Such a focusing grating coupler enables a guided optical wave to be radiated out of an optical waveguide and simultaneously to be focused into a single spot, i.e. it possesses functions (1) and (3). However, it is unable to split the radiated optical wave into a number of optical waves. In other words, a focusing grating coupler cannot simultaneously produce more than one focusing spot. This problem remains to be solved.
In some other applications, such as a computer-generated hologram for use in an optical phase-matched filter, it is generally advantageous to introduce a continuous level phase modulation to an incoming optical wave. so far, most efforts have been concentrated on achieving different phase levels by utilizing a surface-relief stop-like structure (FIG. 1), usually fabricated using a standard lithography and etching technique. As the number of required phase levels increases, so do the number of lithographic masks and the number of etching steps. This indicates that it is difficult and costly to fabricate computer-generated holograms with a larger number of phase levels, because each new mask must be precisely aligned with the previously etched pattern and the height of each step must also be accurately controlled in order to provide the desired phase shift. Therefore, it is beneficial to establish a method which is able to provide multi-level or even continuous level phase shifts using only two-level (or binary) surface-relief structures.
Also in the prior art is a computer-generated guided-wave holographic structure proposed by Saarinen et al. (see for example, Saarinen et al, "Computer-generated guided-wave holography; application to beam splitting", Optics Letters, Vol. 17, No. 4, Feb. 15, 1992). In this device, both the incoming optical wave and the output optical waves are confined with the optical waveguide. The proposed holographic structure comprises a two-level surface relief with a mathematically synthesized geometric shape extending along the surface of the optical waveguide. The phase modulation is achieved through effective refractive index modulation realized, e.g. by means of a patterned cover layer with the mathematically synthesized geometric shape. As a result, the phase shift is directly related to the product of the value of the effective refractive index modulation and the path length the guided optical wave travels through. Using the conventional fabrication technology, however, it is very difficult and often costly to accurately predict the effective refractive index modulation (see Saarinen et al, "Computer-generated guided-wave holography; application to beam splitting", Optics Letters, Vol. 17, No. 4, Feb. 15, 1992). Consequently, the resulting phase modulation may deviate from the desired value, thereby leading to degradation of the device performance. This problem remains to be solved.
Optical storage arrangements, e.g. holographic sheets which preferably have been computer-generated, are commonly used nowadays in several applications, for example in data storage, coherent laser beam addition, free-space interconnections, laser beam shaping, etc. According to the difference in signal encoding techniques, it is possible to categorize computer-generated holograms into two groups; amplitude-modulated holograms, and phase-modulated holograms. The phase-modulated hologram is usually preferable over the amplitude-modulated hologram, because the former can provide a higher diffraction efficiency. Binary phase holograms, i.e. two phase-level holograms, are relatively easy to fabricate, but their application is limited due to low diffraction efficiency. According to the scalar diffraction theory, binary phase holograms may have a theoretical maximum diffraction efficiency of 41%. In order to increase the diffraction efficiency, multilevel or continuous level phase-modulated holograms are often more desirable.
Moreover, according to the characteristics of the input and the output waves, computer-generated holograms can also be categorized into the following three groups: Free-space-to-free-space, guided-wave-to-guided-wave and guided-wave-to-free-space (or free-space-to-guided-wave). Each of these groups are shown as examples in FIGS. 1a-1c. FIG. 1a shows an example of a free-space computer-generated hologram FS-CGH, which belong to the group free-space-to-free-space, where no guided wave is involved. The arrows represent an optical wave which passes through the free-space computer-generated hologram and is projected on a screen. FIG. 1b shows an example of a in-plane computer-generated waveguide hologram IP-CGWH, which belongs to the group guided-wave-to-guided-wave. In the in-plane computer-generated hologram, both the input and the output waves are confined within an optical waveguide OW. The arrows represent an optical wave which passes through the in-plane computer-generated waveguide hologram and is projected in front of one edge of the hologram. FIG. 1c shows an example of an off-plane computer-generated waveguide hologram OP-CGWH, which belongs to the group guided-wave-to-free-space.
In the off-plane computer-generated hologram, both the input and the output waves are confined within an optical waveguide OW, and one of the involved waves may be inputted from or outputted to the free space. The arrows represent an optical wave which passes through the off-plane computer-generated waveguide hologram and is projected in free space.
Free-space computer-generated holograms have been most widely used in applications such as coherent laser beam addition, array generation, laser beam reshaping, etc. However, presently there is a lack of a generally applicable scheme which can be utilized to design and fabricate such holograms. The same problem as in free-space computer-generated hologram may occur for the other two types of holograms, i.e. in-plane and off-plane computer-generated waveguide holograms.
It is well-known that an optical storage arrangement may be composed of an array of rectangular cells which introduces amplitude and/or phase modulation to an incoming optical wave. Most efforts have been concentrated on achieving different phase levels by utilizing surface-relief step-like structures, usually fabricated using a standard lithography and etching technique. The number of achievable phase levels is usually equal to the number of surface-relief step levels. However, as the number of desired phase levels increases, so do the number of lithographic masks and the number of etching steps. This is both difficult and costly, e.g. when producing a hologram with a large number of phase levels, because each new mask should be precisely aligned with the previously etched pattern(s) and the height of each step must also be accurately controlled in order to provide the desired phase shift. Further, it is difficult to control the most commonly used etching techniques in a accurately manner. As a result, the shape and the depth of the relief can differ from its desired value, which can lead to a reduction of diffraction efficiency and/or poor repeatability of performance.
In addition, many optical storage arrangements are either amplitude-modulated or phase-modulated, which will restrict the design freedom of the arrangements. Further, it is very hard to simultaneously control the intensity and the phase of the diffraction space, thereby limiting the application of optical storage arrangements.
In some applications such as data storage, it is desirable to store a large number of images in a common volume of an optical storage arrangement, e.g. of a holographic material, in order to build up a desired storage capacity. An optical storage arrangement that contains more than one image is usually called "multiplexed". A multiplexed optical storage arrangement can be achieved, for example, (1) by directing the replaying optical beam at a particular angle, i.e. "angular multiplexing"; (2) using replaying beams with different spatial phase modulation, i.e. "phase code modulation", or (3) using replaying beams with different wavelength, i.e. "wavelength multiplexing". In case of multiplexing it may be difficult to replay the stored images independently, using guided waves incident from different locations, without introducing excessive crosstalk.